Impedance - Beam RST100 Configuration Manual

RST-100 INSTALLATION AND CONFIGURATION MANUAL

Impedance

Impedance settings relate to the load the RJ11 phone presents to the RST. As the RJ11
phone line interface is a 2-wire standard, it relies on careful balancing between the
impedance expected and experienced by the RST to stop signals in each direction being
improperly reflected.
In normal telephone installations this is not that critical because a reflection at the far end is
just experienced by the near end user as an increase in side tone (or how much you hear
your own voice).
In the Iridium application, however, the echo reflected back to the far end user is delayed
twice by the satellite and landline to USA delays. By the time the user ringing the satellite
phone gets to hear his voice back again it has been delayed up to a second, and this can be
quite annoying.
An incorrect line impedance setting will mainly merely affect the levels, but incorrect hybrid
balance impedance will directly increase the amount of echo.
Most phones will nominate the impedance on their labels and will either be 600 Ohm or
TN12. TN12 impedance is mandated for any phones approved for use on the Australian
network. Most phones used in US will be 600 Ohm. Some phones are meant to be a
nominated impedance but simply aren't. In this case choose the setting that works best and
then reduce the downstream gain till the far end echo is acceptable.
The impedance can be changed dynamically in a call and has an effect immediately.
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